1. Field of the Invention
The present invention relates to an alignment system of a lithography apparatus, and more particularly, to an improved alignment system of a lithography apparatus in which an alignment position corresponds to a light exposing position.
2. Description of the Background Art
Generally, a wafer aligning method includes two methods. One is a method where an alignment light is incident from an alignment system of a lithography apparatus, the incident is light is diffracted, and the diffracted light is collected from an alignment mark on a wafer. The other is an image processing method where the light diffracted from the alignment mark is formed as an image on a charge coupled device (CCD) and the converted electrical signal is processed.
FIG. 1 is a view showing a construction of an alignment system of a lithography apparatus with the method using a diffraction light and the image processing method according to the conventional art. An alignment system for adopting the method using a diffraction light includes a wafer stage (not illustrated) having a wafer 3 mounted thereon, a helium-neon laser 7 for emitting a laser beam which is a coherent beam serving as a light source, a plurality of beam splitters 4 for reflecting a portion of the laser beam and transmitting another portion thereof, an alignment mark 10 mounted on the wafer 3 for reflecting the beam incident on the wafer 3 from the beam splitter 4 and an objective lens, a light collecting device 9 for collecting the beam when the reflected beam is splitted in the beam splitter 4 and passes through a spacial filter 11, and a signal processor (not illustrated) electrically connected to the light collecting device 9. The signal processor is electrically connected to the wafer stage.
The image processing method includes a halogen lamp used as a light source for emitting a coherent beam, a CCD camera 6 for converting a light signal into an electrical signal after detecting an incident diffraction light and outputting the electrical signal to a digital image processing system 8. The digital image processing system 8 is electrically connected to the wafer stage.
After the wafer alignment is performed as described above, a predetermined amount of light (for example, Ultra Violet:UV) is projected to a light exposure region 12 on the wafer 3 through a reduction projection lens 1 disposed on an upper portion of the wafer 3 to form a desired layer (for example, a conductive layer or an oxide film).
The operation of the alignment system of a conventional lithography apparatus will now be described in detail.
In the method using a diffraction light, a laser beam is emitted from the helium-neon laser which serves as a light source 7. A predetermined portion of the emitted beam is reflected, and the other portion thereof is transmitted through a plurality of beam splitters 4. The beams reflected from the beam splitters 4 are incident on the alignment mark 10 on the wafer 3 through the objective lens 2 to be diffracted. Among the diffracted lights, a zeroth order diffraction light is cut off by the spacial filter 11 during its incidence to the light collecting device 9 through the beam splitter 4, and a first order diffraction light reaches the light collecting device 9 after passing through a slit of the spacial filter 11. Accordingly, a light signal incident on the light collecting device 9 is converted into an electrical signal, and a data based on the converted signal is processed in the signal processor (not illustrated), and fed back to the wafer stage to serve as a position information signal of the wafer 3. Then the alignment of the wafer 3 is carried out.
In the digital image processing method, a coherent beam is emitted by using a halogen lamp (not illustrated) which serves as a light source. Among the emitted beam, a portion thereof is reflected through the plurality of beam splitters 4 and the other portion thereof is transmitted. The reflected beampasses through the objective lens 2 to be reflected on the alignment mark 10 on the wafer 3, and the reflected beam is formed as an enlarged image on the CCD camera 6 after passing through the beam splitter 4 to be converted into the electrical signal and applied to the digital image processing system. The digital image processing system uses a pattern matching method to process an inputted signal, and the processed signal is fed back to the wafer stage (not illustrated) to be used as a position information signal of the wafer 3, which is the completion of the alignment of the wafer 3.
However, according to the conventional alignment system of a lithography apparatus and the conventional method, since the positions of the alignment mark 10 and the light exposed region 12 for the alignment of the wafer 3 are different from each other, a base-line error occurs due to a mechanical and thermal drift of the light exposing apparatus, resulting in the lowering of a precision of the wafer alignment.